1. Field of the Invention
The present invention relates to the field of semiconductor wafer processing using a linear polisher for chemical mechanical polishing of the semiconductor wafers. More particularly, the present invention relates to aligning a linearly moving pad and belt combination on a roller used in the linear polisher.
2. Description of the Related Art
The manufacture of an integrated circuit device requires the formation of various layers (both conductive and non-conductive) above a base substrate to form components and interconnects. During the manufacturing process, it is often necessary to remove a certain layer or portions of a layer in order to pattern and form the various components and interconnects. Chemical mechanical polishing (CMP) is one technique for planarizing a surface of a semiconductor wafer, such as a silicon wafer, at various stages of integrated circuit processing. Other uses of CMP include planarizing optical surfaces, metrology samples, and various metal and semiconductor based substrates.
CMP typically uses a chemical slurry along with a polishing pad to polish away the specified materials or layers on a semiconductor wafer. The mechanical movement of the pad relative to the wafer in combination with the chemical reaction of the slurry disposed between the wafer and the pad provide the abrasive force with chemical erosion necessary to polish the exposed surface of the wafer (or a layer formed on the wafer) when subjected to a force pressing the wafer to the pad. In the most common method of performing CMP, a substrate mounted on a polishing head rotates against a polishing pad placed on a rotating table (see, e.g., U.S. Pat. No. 5,329,732). The mechanical force for polishing comes from the rotating table speed and the downward force on the head while the chemical slurry constantly flows under the polishing head. Rotation of the polishing head helps in the slurry delivery as well as in averaging the polishing rates across the substrate surface.
One technique for obtaining a more uniform chemical mechanical polishing rate is to utilize a linear polisher. Instead of a rotating pad, a belt with a coupled pad moves linearly across the wafer surface. It is still necessary to rotate the wafer for averaging out the local variations; however, the linear polisher has a better global planarity over CMP tools using rotating pads. One example of a linear polisher is described in a pending application titled "Linear Polisher And Method For Semiconductor Wafer Planarization;" Ser. No. 08/287,658; filed Aug. 9, 1994.
A major problem with using a linear polishing tool for CMP is keeping the pad/belt combination (hereafter simply "pad/belt") aligned on the rollers. FIG. 1 illustrates a prior art linear polishing tool 10 with a pad/belt 15. The polishing tool 10 comprises the spindles, 13 and 14. Coupled to each spindle are the rollers 32 and 34, and coupled to each roller are the roller pads 33 and 35. The alignment of the pad/belt on the rollers is very important in the uniformity of the wafer polishing. A small variation (or tolerance) in the alignment of the pad/belt on the rollers is acceptable and is currently about 1/16 inch or less, although the tolerance may increase or decrease due to a number of factors. A variation greater than the allowed tolerance, however, will produce an uneven planarization of the wafer, which could produce inoperable devices from the wafer (resulting in low die yields from the wafer). A larger variation in the alignment could even cause the pad/belt to miss or not planarize parts of a wafer. And in a worst case situation, a very large variation in the alignment could even cause the pad/belt to come off of the rollers, producing disastrous consequences to the semiconductor wafer and the tool itself.
A pad/belt generally will self align only when a number of factors are met. Some of these factors include the following: the spindles and rollers are perfectly parallel to each other, the pad/belt is perfectly flat (that is the pad/belt has no arch or sag in the middle of the pad/belt), and the circumference of both sides of the pad/belt are the same. Unfortunately, these factors are difficult to meet due to a variety of other factors including: the manufacturing of the pad and the belt and coupling them together, the high linear operating speed of the pad/belt, and the various forces applied to the pad/belt during the planarization process of the wafer. One example is the large shear and tensional forces applied to the pad/belt by the rotating polishing head with the attached wafer. Another example occurs during the manufacturing of the pad and or the belt that may introduce small variations into their respective surfaces or in their respective circumferences. And, another example is during the coupling of the pad to the belt if the respective circumferences are not within tolerance. The present invention overcomes the above problems by disclosing an apparatus and method for aligning a pad/belt on a roller used in a linear polishing tool used in CMP.